Most commonly employed processes for producing a seamless pipe include the Mannesmann-plug mill process and Mannesmann-mandrel mill process. In these processes, a solid billet heated to a predetermined temperature in a furnace is pierced by a piercing-rolling mill to be formed into a hollow, bar-shaped hollow piece, which is then reduced mainly in wall thickness by an elongator such as a plug mill or a mandrel mill to be formed into a hollow shell. Then, the hollow shell is reduced mainly in outside diameter by a reducing mill such as a sizer or a stretch reducer to be formed into a hot finished seamless pipe of a predetermined size. The present invention relates to a method for producing a seamless metal pipe, the method including producing a thin-wall hollow piece particularly by piercing-rolling a billet made of a less formable material at a high reduction rate in the first step of piercing-rolling among the above-mentioned steps.
First of all, inventions that have been proposed by the present inventor and others in Patent Literatures 1 to 4 will be described as conventional techniques.
The invention of Patent Literature 1 (hereinafter referred to as the first prior invention) is a method in which piercing-rolling is performed in such a manner that a feed angle β of cone-type main rolls supported at both ends and arranged horizontally or vertically to face each other across the pass line along which the billet or the hollow piece passes and a cross angle γ of the main rolls are maintained to be within the ranges defined by the following Formulae (1)′ to (3)′, with the billet or the hollow piece being pressed by the surfaces of disc rolls arranged vertically or horizontally to face each other across the pass line between the main rolls.3°≤β≤25°  (1)′3°≤γ≤25°  (2)′15°≤β+γ≤45°  (3)′
The feed angle β is an angle of the roll axis line with respect to a horizontal plane or a vertical plane of the pass line, and the cross angle γ is an angle of the roll axis line with respect to a vertical plane or a horizontal plane of the pass line.
The first prior invention fundamentally negates the piercing principle of the Mannesmann piercing process. The conventional Mannesmann piercing process is a piercing-rolling process in which a solid billet is pierced utilizing the so-called rotary forging effect (Mannesmann effect) to create a condition that facilitates piercing, whereas the first prior invention is based on the technical ideas of:
(i) inhibiting the occurrence of the rotary forging effect (Mannesmann effect) as much as possible; and
(ii) inhibiting circumferential shear deformation γrθ and shear deformation γβ1 due to surface twist which occur during the piercing process as much as possible to realize a metal flow comparable or nearly comparable to that of the extrusion pipe-making process when it is inclined rolling.
To achieve the purpose, the piercing rolling mill is configured so as to enable high cross angle and high feed angle piercing, with the shape of the main rolls being of the cone type and disc rolls being employed instead of guide shoes.
The invention of Patent Literature 2 (hereinafter referred to as the second prior invention) is a method for producing a seamless pipe in which: a feed angle β of cone-type main rolls supported at both ends and arranged horizontally or vertically to face each other across the pass line along which the billet or the hollow piece passes and a cross angle γ of the main rolls are maintained to be within the ranges defined by the following Formulae (1) to (3); the diameter do of the solid billet and the outside diameter d and wall thickness t of the hollow piece after the piercing-rolling satisfy the following Formula (4); and the piercing ratio is 4.0 or more, the pipe expansion ratio is 1.15 or more, or the “wall thickness-to-outside diameter” ratio is 6.5 or less.8°≤β≤20°  (1)5°≤γ≤35°  (2)15°≤β+γ≤50°  (3)1.5≤−Ψr/Ψθ≤4.5  (4)                where Ψr=ln(2 t/d0)                    Ψθln {2(d−t)/d0}                        
The second prior invention described above, similarly to the first prior invention, is a method designed to inhibit, as much as possible, the rotary forging effect and redundant shear deformation, which significantly occur in a piercing-rolling step, particularly a thin-wall piercing-rolling step at a high reduction rate, by maintaining the feed angle β and cross angle γ of the rolls to be within a suitable range. In addition, the method is designed to prevent inner surface flaws and laminations (cracks that can occur in the wall thickness central portion) that can occur in production of stainless steel pipes or high alloy steel pipes and further to reduce operational troubles such as pipe wall flaring, pipe wall peeling, and tail clogging by optimizing the distribution of the circumferential strain Ψθ and thicknesswise strain Ψr so as to satisfy the relationship represented by Formula (4). Here, it is to be noted that, in the second prior invention, Formula (4) means that, for accomplishing high reduction rate thin-wall piercing, a high piercing ratio piercing process is not selected but a high pipe expansion ratio piercing process is employed.
In view of what is written in Claims, the first prior invention is not necessarily limited to the pipe expansion piercing process solely but the second prior invention is clearly limited to high pipe expansion ratio piercing.
The above two prior inventions imply that, in order to stably pierce a less formable material such as a stainless steel or a high alloy steel without causing inner surface flaws or laminations, the roll gorge diameter should be as small as possible relative to the billet diameter. However, reduction of the roll gorge diameter requires, in light of the roll structure, that roll shaft diameters at the entry side and the exit side also be reduced. Then, the strength of the bearing that supports the roll shaft would be insufficient, and particularly in the case of a cone-type roll, the fatigue strength of the bearing at the entry side would be insufficient, leading to the problem of durability. Thus, excessive reduction of the roll gorge diameter is not recommendable for actual operation.
Next, the object of the invention of Patent Literature 3 (hereinafter referred to as the third prior invention) is to provide a piercing-rolling method capable of inhibiting the rotary forging effect as much as possible and inhibiting redundant shear deformation as much as possible while avoiding excessive reduction of the roll gorge diameter.
As described above, the present inventor proposed a high cross angle expanding-piercing-rolling process in order to kill the rotary forging effect and inhibit redundant shear deformation, and thus made the second prior invention. However, although enlargement of the cross angle is a necessary condition for killing the rotary forging effect and inhibiting redundant shear deformation, it is not a sufficient condition. The necessary and sufficient condition is optimization of the roll shape while enlargement of the cross angle is a necessary condition for optimizing the roll shape.
In the piercing-rolling method of the third prior invention, the relative relationship between the pipe expansion ratio of the pipe material and the diameter expansion ratio of the cone-type main rolls is optimized. As a result, the rotary forging effect during piercing-rolling is significantly inhibited, and thus it is possible to more reliably inhibit inner surface flaws and laminations, which are likely to occur during the process of high reduction rate thin-wall piercing-rolling of a less formable material such as a stainless steel or a high alloy steel.
In the third prior invention, in addition to the above-mentioned (1) to (4), the following formulae (5) and (6) defining the relationship between the inlet diameter D1 of the main roll, the outlet diameter D2 thereof, the diameter do of the billet, the diameter d thereof after the piercing, and the cross angle γ are further satisfied.(d/d0)/(0.75+0.025γ)≤(D2/D1)  (5)D2/D1≤(d/d0)/(1.00−0.027γ)  (6)
When discussing the relationship between the pipe expansion ratio “d/d0”, the roll diameter expansion ratio “D2/D1”, and the roll cross angle γ, whether the roll shape is suitable or unsuitable needs to be determined by the rotary forging effect, and here, the determination criterion is whether the ductility (reduction value) of the central portion of the billet immediately before being contacted by the plug tip can be made greater than the reduction value of the billet itself. The above Formula (5) is an essential requirement for specifying the roll shape, but Formula (6) is not necessarily a requirement because, in many cases, it is satisfied unintentionally.
The invention of Patent Literature 4 (hereinafter referred to as the fourth prior invention) is an invention relating to a technique of installing disc rolls, but it is not described here because, in the present invention, disc rolls are not used as detailed below.